In forming films by using plasma Chemical Vapor Deposition (CVD) devices, it is important to suppress generation of fine particles called powder (for example, see Patent Literature (PTL) 1: Japanese Unexamined Patent Application Publication No. 2004-296526). The suppression of power generation requires measurement of a substrate temperature. However, it is difficult to directly measure the substrate temperature. In order to address this, a ratio of light emission intensities between a silicon* (Si*) film and a silan* (SiH*) film (where “*” represents an atomic valence) which are formed above the substrate is calculated as a value related to the substrate temperature. Such a conventional plasma CVD device controls a gas flow rate in the plasma CVD device to keep the calculated ratio constant, thereby suppressing the powder generation.